Species multiply as Earth hots up

Rather than kicking off extinction cycles, as some scientists had previously believed, warming cycles in the Earth's history are actually accompanied by increased biodiversity, a new study[1] finds.

The paper, in this week's Proceedings of the National Academy of Sciences, compared the number of known genera of marine invertebrates with sea-surface temperatures in 51 eras covering 540 million years of the Earth's history. It found that when temperatures were high, so was biodiversity. When temperatures were lower, biodiversity declined as well.

The finding reverses prior work, particularly from lead author Peter Mayhew's own group,[2] that had found an inverse correlation between high temperatures and biodiversity.

The reason for the change, says Mayhew, an evolutionary ecologist at the University of York, UK, is that the earlier work measured fossil diversity by tallying first and last appearances of each group of species, then presuming the creatures existed only during the intervening years.

That sounds logical but overlooked the fact that some geological periods are better studied than others -- meaning that just because an organism first appears at one date and last appears at another doesn't mean it wasn't actually present before and after, especially if the first and last appearances are in particularly well-studied eras. "The sampling frequency is going to cause a bias," Mayhew says.

To correct this, the new study looked only at the well-sampled periods. And instead of interpolating organisms' presence from origination and extinction dates, it merely tallied species groups present in each period. "That is how you would do it if you were an ecologist," Mayhew says.

Even so, given the fact that climate change is generally viewed as disruptive, it was a "big surprise" to find that warming eras were accompanied by increases in biodiversity. (On the other hand, Mayhew notes, it has long been known that tropical ecosystems are the most diverse, so it had previously been something of a conundrum that warm eras, when the tropics should have expanded, appeared to be comparatively species-poor.)

What appears to be happening, Mayhew says, is that warming produces both extinctions and originations, with the originations of new species outstripping the loss of old ones.

This does not mean, however, that today's climate change is a good thing.

"The rate of change is very important," Mayhew says. "For diversity to rise, you need speciation to happen." And that, he says, takes thousands to millions of years – much longer than the pace at which extinctions are likely to occur with today's rapid change.

Scott Wing, a paleobiologist, at the Smithsonian Institute, Washington, D.C., agrees. "Don’t report this as global-warming news," he says. "This article has nothing to say about the effects of global warming at any time scale of interest to most humans."

That said, he adds, "this is definitely of interest to evolutionary biologists, paleontologists, and ecologists seeking to understand very broad patterns of diversity."

Shanan Peters, a paleobiologist at the University of Wisconsin-Madison, however, isn't so sure the paper is groundbreaking. Its primary result, he notes, is to overturn Mayhew's own prior finding and bring the long-term diversity results into line with ecological common sense. "Paleobiologists and climatologists have long referred to warm intervals as 'climate optima,'" he notes, "precisely because it is during such times that palm trees and alligators inhabit the arctic and life appears to be diverse and flourishing."

More interesting, he says, will be the next generation of results, when paleobiologists turn their attention to major climate transitions, such as may be occurring now.

Mayhew agrees. "The time periods we're really interested in now," he says, "are decades and hundreds of years – at maximum 1,000 years."

Studying those, he adds, will require something other than a 540-million-year time horizon. "You can't get that kind of detail by looking into the deep past," he says. "If you want to know how temperature change is affecting things on that time scale, you're going to have to look at the more recent fossil record."